Mechanical cascade refrigeration systems are well-known in the art for providing various desired levels of refrigeration. In the event that substantially constant refrigeration levels must be maintained, the refrigeration system must have some form of emergency back-up protection. A duplicate mechanical refrigeration system may be employed as back-up, but this is generally quite expensive.
Liquid nitrogen cooling systems are also known in the art. In one form of liquid nitrogen cooling system, the liquid nitrogen is sprayed or atomized into the chamber being cooled. While this provides rapid cooling, it has several disadvantages. First, the liquid nitrogen is at a temperature of about -320.degree. F. (-196.degree. C.). If it comes in direct contact with various materials, such as the material being cooled or the gaskets of the cooling chamber, damage may occur to such materials. Second, the gaseous nitrogen resulting from evaporation of the liquid nitrogen is an inert substance which can displace air from the cooling chamber. This can be hazardous to any humans or animals present in the cooling chamber. This form of refrigeration is not suitable for long term primary maintenance of cooling chambers because of the expensive consumption of liquid nitrogen. If this form of liquid nitrogen cooling is employed as a back-up to a mechanical refrigeration system it has several disadvantages. It will have the above inherent disadvantages and will also require separate cooling apparatus connections to the chamber being cooled.
In another form of liquid nitrogen cooling system a chiller or heat exchanger can be employed wherein liquid nitrogen on one side of the heat exchanger can cool a refrigerant on the other side of the heat exchanger. Here again, this type of apparatus is not suitable for long term primary maintenance of cooling chambers because of the expensive consumption of liquid nitrogen. If this form of liquid nitrogen cooling is employed as a back-up to a mechanical refrigeration system, it has several disadvantages. It will require duplicate refrigerant circulation systems with duplicate pumps and conduits connected to the cooling chamber. This adds to the overall expense. Since the refrigerant in the back-up system will normally be at room or ambient temperature, there will be a delay in getting the back-up system to desired low temperature. The liquid nitrogen will be employed only when needed as back-up and it will first need to cool the refrigerant.
There is thus a need for a quick-acting liquid nitrogen-cooled back-up refrigeration apparatus available at minimum expense.